In recent years, CIS-based solar cells having a structure provided with a photoelectric conversion layer constituted of compounds of metals such as copper (Cu), indium (In) and selenium (Se) (CIS-based thin films) as well as a lower electrode layer and an upper electrode layer, unlike conventional crystalline silicon based solar cells, are attracting attention as new generation solar cells. Since there are advantages associated with the CIS-based solar cells such as their simple structure, the ease of making them into large solar area cells, the simple production process therefor and their energy saving potential, various studies have been conducted and various suggestions have also been made.
In order to improve the performance of CIS-based solar cells, alkali metals such as sodium (Na) need to be added to the photoelectric conversion layer. In those cases where soda lime glass or the like which generally includes soda lime (Na2O.CaO.5SiO2) as a major component thereof is used as a substrate, there is no need to intentionally add alkali metals since the alkali metals contained in the substrate diffuse into CIS metals constituting the photoelectric conversion layer. On the other hand, in those cases where alkali-free glass or low alkali glass excellent in heat resistance is used as a substrate or cases where a metal foil substrate or a substrate made of plastic such as polyimide is used for the sake of preparing a flexible solar cell, since the diffusion of alkali metals cannot be expected from the substrate, it is necessary to diffuse alkali metals into the CIS-based thin films by using an alkali precursor.
In these cases, it is common to use sodium fluoride (NaF) or the like as an alkali precursor, and various methods have been employed, such as a method in which the precursor is deposited on top of a lower electrode layer (Non-Patent Document 1), and a method in which the precursor is deposited, following formation of a CIS-based photoelectric conversion layer, on top of the photoelectric conversion layer and is then diffused thereinto by a heat treatment (Non-Patent Documents 2 and 3).
[Non-Patent Document 1] V. Probst, et al., Advanced stacked elemental layer process for Cu(In,Ga)Se2 thin film photovoltaic devices, MRS Symp. Proc., Materials Research Society, vol. 426 (1996) p. 165
[Non-Patent Document 2] D. Rudmann, et al., Efficiency enhancement of Cu(In,Ga)Se2 solar cells due to post-deposition Na incorporation, Applied Physics Letters, American Institute of Physics, vol. 84, p. 1129 (2004)
[Non-Patent Document 3] D. Rudmann, et al., Na incorporation into Cu(In,Ga)Se2 for high-efficiency flexible solar cells on polymer foils, Journal of Applied Physics, American Institute of Physics, vol. 97, p. 084903-1-5 (2005)